The present invention relates to methods of obtaining a semiconductor body from a semiconductor polymeric compound and to the semiconductor body obtained from that method.
Polymeric semiconductor compounds are already known including, essentially, plastic materials which incorporate electrically conducting materials. These materials are widely used, but a major difficulty effecting their performance has been the electrical insulation of the particles of the conducting materials caused by the plastic material which results unless a high concentration of the conductive particles is used. High concentrations of these conducting particles are not ideal, since they can jeopardize the desirable mechanical properties of the polymer compound, making it hard and fragile.
Thus, the art of making a good polymeric semiconductor compound consists in using a minimum concentration of electrical conducting material to obtain the desired degree of electrical performance.
Lampblack, which may be obtained from combustion or cracking of hydrocarbons and consists essentially of carbon, is outstanding in conducting loads. Lampblack is classified as an inorganic material, in spite of the fact that it consists of carbon, is compatible with polymeric matrices and has a low cost.
In spite of the large acceptance of lampblack in polymeric semiconducting materials, it has many difficult "handling" problems in the process of making a semiconductor. These problems relate to the lampblack structure itself, its concentration and distribution in the polymer, its ingredients and methods of mixing of them. They also relate to the final molding of the semi-conducting part or body. They also impair the electrical characteristics of the product that is obtained.
In view of the foregoing problems and a strong interest in obtaining a highly efficient polymeric semiconductor compound for several devices, especially those used in the automobile industry, a series of studies on the structure and method of making this material and its relationship to the final conductivity of the material were developed. These studies are based on the existing technical literature and mainly on exhaustive laboratory experiments.
The theory of electrical conductivity of plastics loaded with lampblack particles shows that first of all the resistivity of the lampblack carbon particles should be considered.
For lampblack to be conductive the lampblack particles should have a comparatively small particle size, large structure, high porosity, and low content of volatile compounds, among other properties.
Another important fact effecting the conductivity of the lampblack is the chain structure and chain length. Recently, it was observed that, actually, the width or gaps between chains is of greater importance than the length and should be limited to allow electron tunneling, that is, to allow electrons to "jump" from one chain to another. When the gaps have widths which are larger than those suitable, the flow of electrons is interrupted and the material changes from a conductive to a nonconductive material.
Another significant parameter effecting the conductivity of lampblack in the polymeric semiconductor is the concentration, since the conductivity of the polymer semiconductor compound and lampblack do not increase linearly with the increase of lampblack concentration.
Indeed, studies show that, up to certain limits, a small increase in the concentration of lampblack in the semiconductor results in a considerable increase in conductivity. However above those limits, a great increase in the concentration does not provide a correspondingly great increase in conductivity, this phenomenon being known as percolation, with a large surface area or many pores of the lampblack particles corresponding to a low percolation number.
On the other hand, the functional relationship between resistivity and lampblack concentration has proved difficult to determine. The method of mixing in the manufacture of the compound is of great importance.
Several studies have been made in an effort to overcome the difficulties involved in understanding these materials. For instance, an early theory of Voet, Whitter and Cook, in 1965, proved that systems that show non-ohmic conduction in low conductivity composites can have electron tunneling occurring over gaps of up to 5 nm. The Brownian movement of the comparatively small lampblack particles is enough so that the particles approach close enough for the electron tunneling to occur.
A second theory of Scardisbrick of 1973, involves high conductivity ohmic conduction, and implicitly assumes that contacts between particles are ohmic, calculates probability of formation of random conductivity changes, considering the sphericity of the particles and other factors.
Besides the aspects mentioned above, which relate to the relationship of conductivity to the specific properties of the lampblack, other facts have an outstanding effect on the conductivity, for example, the processing method.
Thus, during the making of the semiconductor compound, it has been observed that the level of dispersion is critical. If the mixture is not uniform, there is a loss of the lampblack features which are important for conductivity such as chain structure, surface area and porosity and/or lack of dispersion. On the other hand, in the case of high shear during mixing, the mixture will be uniform. Both extremes, however are unfavorable for improvement of conductivity.
From that it can be concluded that the shearing and dispersion conditions in several molding methods, such as: extrusion, calendering, pressing, or others, are causes of different conductivities in mixtures of identical composition.
Furthermore, the method of processing a mixture of lampblack/polymer has a significant effect on the orientation of the particles and, consequently, on the conductivity properties of the finished product.
This relation is not only the result of reduction of lampblack structure with corresponding favorable distribution, but also of increase of orientation of lampblack aggregations during processing.
Thus, for the same composition, pressed blades show small specific resistance, while extruded sections, injected disks and tubular films show an increase of specific resistance. Articles molded by injection and consisting of thermoplastics and lampblack show a significant reduction in the specific resistance after a period of three months of storage, while semicrystalline thermoplastics retain the specific resistance at relatively high levels.
From that it can be concluded that the reduction in lampblack structure causes an increase of specific resistance when pressed and injected articles of the same composition are compared.
Besides, electron microscope studies show that the lower degree of conductivity in articles molded through injection, is due to a larger degree of orientation of lampblack aggregations. The volumetric resistivity in non-oriented formations is lower than in an oriented formation.
Experience shows that the conductivity of the extruded material is adversely influenced by thread speed, geometry and extruded matrix dimensions.
The geometry and dimensions of the matrix have great influence on the conductivity and the thread speed has a secondary influence.
Thus, based on the studies which were summarized above, development of a suitably improved polymeric semiconductor based on lampblack and method of obtaining it was undertaken to obtain electrical materials and/or devices having suitable electromechanical properties for a variety of applications.